When piping is installed in a large plant such as a nuclear power plant, residual stress of tension generated in the inner peripheral surface of the piping poses a problem. When pipings are connected by welding, for example, residual stress occurs in the weld zone of the pipings, and stress-corrosion cracking (SCC) is caused to the piping by the residual stress, potentially shortening the life of the piping. Thus, it is desirable to reduce the residual stress generated in the piping by welding or the like.
Japanese Patent Application Laid-Open No. 2001-150178 (Patent Document 1) discloses an apparatus for improving residual stress of piping, which is designed to reduce residual stress in the vicinity of a weld zone of piping by heating. This apparatus, which is publicly known, is equipped with an arc generating ring located on the outer periphery of piping, and a first ring coil and a second ring coil arranged on the outer periphery of the piping above and below the arc generating coil, respectively. When a magnetic field is generated by the ring coils, an arc occurs between the arc generating ring and the piping to heat the piping. Since the piping is heated, the residual stress of the piping is reduced.
Japanese Patent Application Laid-Open No. 1996-19881 (Patent Document 2) discloses a technique for surface machining the inner surface of piping by irradiating the inner surface of the piping with laser light. According to this technique, which is publicly known, laser light is guided into the piping by an optical fiber, whereafter the laser light is delivered from the optical fiber, and directed at the inner surface of the piping.
A method of stress removal by high frequency heating is widely known. However, the method using high frequency heating has the problems that it involves a large-scale apparatus, supply power for a high frequency wave is very great, and the interior of target piping needs to be cooled.
One of requirements of an apparatus for improving the residual stress of piping, which is used for removing residual stress, is to be able to heat a wide range of the surface of the piping uniformly. The ability to heat a wide range of the surface of the piping is important for increasing a throughput and ensuring residual stress removing performance. The ability to heat the piping uniformly, on the other hand, is important to render residual stress, which remains after heat-treatment, low. Fulfilling this requirement is not easy if the piping has a complicated shape, for example, if the piping is branched, or if a thick-walled pipe is targeted and, particularly, its broad range needs to be heated.
Against such a background, there is a demand for the provision of an apparatus for improving the residual stress of piping, which can uniformly heat a wide range of the surface of piping and, especially, can uniformly heat a wide range of the surface of piping even if the piping has a complicated shape, such as a T-piping.
Patent Document 1: Japanese Patent Application Laid-Open No. 2001-150178
Patent Document 2: Japanese Patent Application Laid-Open No. 1996-19881